CustomFieldAdminPlugin: Adding detection of custom fields provided by code and not as regular trac.ini settings - ie using Option() in plugins. The fields can be updated and changed as regular fields, but they cannot be deleted using the web administration.

I've not tested this so hopefully I've not broken anything major.
I used the patch on #5466 as a basis but totally messed about with the code so any bugs are mine.
Added a preference in the admin section for easy management.
Closes #5466 (sorry for the delay and please let me know if this is buggy :))